This invention relates to a sole for shoes, and in particular, to a sole for sports shoes or as an orthotic insert to improve the lateral stability of the shoe while being compliant in response to the forward rolling action of the foot.
For purposes of illustrating the present invention, reference will be made to the running shoe and the requirements of the running shoe. By way of introduction, the normal gait cycle of a runner's foot should be briefly described to better understand the environment in which the present invention is applicable. Before heel strike, the foot is disposed at an upward angle relative to the ground and rotated or twisted outward, commonly referred to as supination. The initial part of the foot to impact on foot fall is the outward portion of the heel. Upon heel strike or impact, the ankle, knee and hip collectively cushion the shock. There is an inward rolling of the foot, a process called pronation. Pronation is the body's way of partially absorbing the vertical impact force of the foot fall. Excessive pronation, whether caused by lateral instability in the shoe or otherwise, increases the likelihood of injury. The degree of pronation also varies from runner to runner. From the heel strike, through pronation, the impact forces and body weight are transferred to the ball or mid portion of the foot. In the lift-off, the toes propel the foot off the ground and the foot twists outward as the knee and hip extend forward into the next gait cycle. During the gait cycle, while the foot is on the ground, the foot goes through a forward rolling action--first heel, then ball and finally toes.
Running shoes are typically constructed with a mid sole that provides impact cushioning. While impact cushioning reduces vertical shock, such cushioning normally contributes to lateral instability which results in more severe and undesired pronation. Lateral stability is desired in many different forms of shoes, particularly sport shoes, with the shoe being compliant to the forward rolling action of the foot.